The present invention relates to an improved magnetooptic recording material, and, more particularly, to improvement in corrosion resistance of a material used in the field of high density magnetooptic recording.
A wide variety of high density magnetooptic recording media have been developed such as those proposed in Japanese Patent Publication Sho.59-108304. One typical example includes a base plate and a thin coat formed on the base plate having a plurality of easily magnetizable columns formed ar right angles with respect to the base plate. In order to write in signals to such a magnetooptic recording medium, the medium is first treated to have a uniform direction of magnetization over the entire thin coat, and the thin coat then is subjected to local laser beam radiation to raise its temperature up to the Curie point. Under this condition, a weak magnetic field of opposite polarity is applied to the medium in order to invert the direction of magnetization at the heated section Thereafter the medium is cooled down to room temperature. In order to read out signals from such a magnetooptic recording medium, the Kerr effect is utilized. That is, the thin coat of the medium is subjected to polarized laser beam radiation and the difference in light intensity of the reflected beams caused by the difference in the Kerr rotation angle is used for detection of the recorded signals.
Among the various magnetic materials recently proposed for use in the field of high density magnetooptic recording, rare earth metal type alloys and transition metal type alloys have attracted the major attention of those skilled in the art. Tb-Fe-Co alloys are one example. These alloys exhibit easy magnetic inversion because of their relatively low Curie points near 200.degree. C. In addition, no magnetic inversion starts at room temperature because of their relatively high coercive force near 240 kA/m. Further, these alloys assure quite high density recording because of their significant recording stability. These merits induce increased use of these alloys in the field of high density magnetooptic recording.
Despite these significant merits, inclusion of Tb in rare earth metal - transition metal type alloys brings about a significant problem because the active Tb is quite vulnerable to oxidation and, as a consequence, the material is very poor in corrosion resistance, in particular in an electrochemical environment which includes Na+ ion and Cl- ion. The material is easily corroded through contact with human sweat. For improvement in corrosion resistance, Al, Ti or Cr is usually added to the alloy. For appreciable improvement, a great deal of these metals must be added to the alloy. Increased addition of these metals, however, tends to greatly lower the Kerr rotation angle and the coercive force of the resultant magnetooptic recording material.